1. Field of the Invention
This invention relates to an impressed current corrosion protection system, for example for the corrosion protection of buried pipelines or tanks or other substrates.
2. Introduction to the Invention
It is well known to protect an electrically conductive substrate from corrosion by establishing a potential difference between the substrate and a spaced-apart electrode. The substrate and the electrode are connected to each other through a power supply of constant sign (DC or rectified AC) and the circuit is completed when electrolyte is present in the space between the substrate and the electrode. In most such impressed current systems, the substrate is the cathode (i.e. receives electrons). However, with substrates which can be passivated, e.g. Ni, Fe, Cr and Ti and their alloys, it is sometimes also possible to use impressed current systems in which the substrate is the anode. In both cathodic and anodic systems, the substrate is often provided with a protective insulating coating; in this case the impressed current flows only through accidentally exposed portions of the substrate. If the system is to have an adequate life, the electrode must not itself be corroded at a rate which necessitates its replacement; this is in contrast to the "sacrificial anodes" which are used in galvanic protection systems. The electrode must also have a surface which is not rendered ineffective by the current passing through it or by the electrochemical reactions taking place at its surface, such as the evolution of chlorine gas.
The electrode and the power supply must be such that the current density at all points on the substrate is high enough to prevent corrosion but not so high as to cause problems such as damage to the substrate (e.g. embrittlement) or disbonding of a protective coating on it. The power consumption of the system depends inter alia on the distance between the various pans of the substrate and electrode. In view of these factors, the theoretically best type of electrode is one which can be positioned so that it is relatively close to all points on the substrate. To this end it may have a shape corresponding generally to the shape of the substrate. Such an electrode is referred to herein as a "distributed electrode".
EP 0067679 describes a distributed electrode, usually a distributed anode comprising a metal, e.g. copper conductive core and a conductive polymeric jacket. EP 0067679 describes a distributed electrode whose electrically active outer surface is provided by an element which is composed of a conductive polymer which is at least 500 .mu.m, preferably at least 1000 .mu.m, thick. The term "conductive polymer" is used herein to denote a composition which comprises a polymer component, and dispersed in a polymer component, a particulate conductive filler which has good resistance to corrosion, especially carbon black or graphite. In particular the electrode comprises a low resistance core electrically surrounded by a conductive polymer composition, wherein the anode is an electrode spaced apart from the substrate, the electrode being in the form of an elongate flexible strip which can be bent through an angle of 90.degree. over a 10 cm radius, the electrode comprising
(1) a continuous, elongate core which is composed of a material having a resistivity at 23.degree. C. of less than 5.times.10.sup.-4 ohm.cm and a resistance at 23.degree. C. of less than 0.03 ohm/meter; and PA1 (2) an element which PA1 (1) a continuous elongate core which is composed of a material having a resistivity at 23.degree. C. of less than 5.times.10.sup.-4 ohm.cm and a resistance at 23.degree. C. of less than 0.03 ohm/meter, PA1 (2) a conductive polymer composition which electrically surrounds the core and is in electrical contact with the core, and PA1 (3) a polymeric jacket surrounding the conductive polymer composition and containing, between it and the conductive polymer composition, a carbon rich material, preferably coke, PA1 (i) resistant to acid to the extent that if a section of the jacket material is immersed in hydrochloric acid of at least 0.01N concentration at 60.degree. C. for 90 days and then subjected to a tensile test, and a load versus elongation curve is plotted from the tensile test, then PA1 (ii) resistant to chlorine to the extent that if a section of the jacket material is immersed in acidified sodium hypochlorite for 90 days, during which time sufficient acid is added to the hypochlorite solution periodically such that chlorine is continually present (i.e. chemical chlorine), and then the said section subjected to a tensile test, and a load versus elongation curve is plotted from the tensile test, then PA1 (a) the maximum load recorded during that test is at least 60%, preferably 70%, more preferably 80% of the maximum load recorded for a load versus elongation curve for a similar section of the same material which has not been subjected to immersion in the said hydrochloric acid, and PA1 (b) the elongation of the said section at the maximum load is at least 60%, preferably 70%, more preferably 80% of the elongation at the maximum load of a similar section which has not been subjected to immersion in the said hydrochloric acid.
(i) is composed of a conductive polymer composition which has an elongation of at least 10%, according to ASTM D1708, PA2 (ii) provides at least a part of the electrochemically active outer surface of the electrode, and PA2 (iii) is in the form of a coating which electrically surrounds the core and is in electrical contact with the core, and which is at least 500 .mu.m thick. PA2 (a) the maximum load recorded during that test is at least 60%, preferably 70% more preferably 80% of the maximum load recorded for a load versus elongation curve for a similar section of the same material which has not been subjected to immersion in the said hydrochloric acid, and PA2 (b) the elongation of the said section at the maximum load is at least 60%, preferably 70%, more preferably 80% of the elongation at the maximum load of a similar section which has not been subjected to immersion in the said hydrochloric acid; and PA2 (a) the maxima load recorded during that test is at least 70%, preferably 80%, more preferably 90% of the maximum load recorded for a load versus elongation curve for a similar section of the same material which has not been subjected to immersion in acidified sodium hypochlorite solution, and PA2 (b) the elongation of the said section at the maximum load is at least 60%, preferably 70%, more preferably 80% of the elongation at the maximum load of a similar section which has not been subjected to immersion in the acidified sodium hypochlorite solution.
The entire disclosure of EP 0067679 is incorporated herein by reference.
Where a conductive polymer based anode as described in EP 0067679 is used alone for cathodic protection, after many years, in extreme environments, some of the carbon of the conductive polymer jacket may be consumed as part of the corrosion protection electrochemical process. Therefore it is also known, for corrosion protection of soil-buried substrates, to use a coke breeze back-fill around the anode. Thus, for example, for protection of a buried pipeline, a trench may be dug in the soil near to the pipeline, and as the elongate conductive polymer based anode is payed off into the trench, it is surrounded by a layer (e.g. about 50 mm thick) of coke breeze, before the top soil is replaced. This process is described for example in "External Pipeline Rehabilitation" by R. John, Pipeline Magazine, October 1990. The coke breeze provides a greater overall anode surface, and also decreases the overall resistance of the system.
It is also known to deliver the coke breeze pre-packaged in a nylon fabric jacket, the jacket serving as a delivery tool for the coke.